This section provides background information related to the present disclosure which is not necessarily prior art.
There are many uses for sodium hypochlorite, commonly known as bleach in industrial, utility, and residential applications. In many large-scale applications, sodium hypochlorite has traditionally been produced on-site by combining chlorine, alkali, and water. Chlorine is conventionally provided as liquefied chlorine gas in portable cylinders or in rail cars. However there are certain risks and costs associated with the handling, shipping, and storage of liquefied chlorine. An alternative is to handling liquefied chlorine is to produce the chlorine or sodium hypochlorite by electrolysis. Direct electrolysis is described in prior art as the conversion of sodium chloride-containing brine to a solution containing of sodium hypochlorite in an undivided electrochemical cell. This process has the advantage of producing sodium hypochlorite without the transport of gaseous chlorine and solutions containing caustic soda. The principal disadvantage of on-site direct electrolysis to make bleach is that high conversion of salt to bleach is not achievable simultaneously with high energy efficiency. Another problem encountered with direct electrolysis is the limited life of electrodes used in the electrolysis. Yet another problem with direct electrolysis is the undesirable formation of chlorate, either by thermal decomposition of hypochlorite solutions or by the electro-oxidation of hypochlorite at the anode.
Another alternative process for the production of sodium hypochlorite is indirect. This process begins with electrolysis of salt to produce chlorine and caustic soda which are later recombined chemically to produce bleach. Indirect electrolysis is typically performed in a membrane-cell electrolyzer, and can achieve high conversion of salt and high energy efficiency. The chlorine and caustic soda co-produced by this means can be combined in a suitable reactor to produce bleach solutions. However, the indirect production of bleach requires substantial investment in equipment, as well as equipment for safely handling gaseous chlorine. The indirect production of bleach is thus unsuitable for many on-site applications at the point of use, but is the preferred means to produce bleach at an industrial scale. Such production is typically optimized based upon proximity to electric power supply and salt availability, so it is typically uneconomic to produce bleach by indirect electrolysis at many locations where it is needed.
Transportation of bleach solutions is limited by the solubility of sodium hypochlorite in water and by the limited stability of these solutions. Transportation cost of bleach solutions of 15-25% concentrations is higher than the cost of transporting the reactants (50% caustic soda and liquefied chlorine gas) used to produce bleach conventionally, because more mass and volume must be transported per unit of sodium hypochlorite delivered.